As is generally known, measurement of the number of particles existing in a clean space such as a clean room is a very important factor in semiconductor production processes. As a prior art, there have been used optical particle counters (OPCs) using a laser, diffusion batteries using Brownian diffusion, or the like in order to measure the size of particles.
As the line widths of semiconductors decrease due to the development of semiconductor technology, particles of about 10 nm in diameter should necessarily be measured. However, the aforementioned optical particle counters cannot measure particles of 0.1 μm or less in diameter due to laser scattering. In addition, when particles are measured using diffusion batteries, there is a problem in that the measurement results are imprecise.
At present, in order to measure particles' of about 10 nm in diameter, differential mobility particle counters have been developed. A differential mobility particle counter comprises a combination of a differential mobility analyzer (DMA) and a condensation particle counter. The differential mobility analyzer selects particles with desired diameters using a particle diameter, particle flow and electrostatic force, and then the condensation particle counter measures the number of particles selected by the differential mobility analyzer using a computer.
FIG. 1 is a view showing an apparatus for measuring the number of particles using the prior art. Referring to FIG. 1, the apparatus for measuring the number of particles of the prior art comprises a particle injector 10, a particle separator 20, and a particle counter 30.
The particle injector 10 is positioned upstream of the particle separator 20. The particle injector 10 includes a particle feeder 12, a neutralizer 14 as a particle charger for charging supplied particles, and a clean air feeder 16. The neutralizer 14 electrically neutralizes particles by charging them to a positive polarity using radioactivity. Since such a neutralizer is well known those skilled in the art to which the present invention pertains, the configuration and operation thereof will be omitted.
The particle separator 20 includes a cylindrical outer guide duct 21, a cylindrical inner guide duct 22, an electrode 23 installed in the inner guide duct 22, and a particle separating duct 24 extending from a lower end of the electrode 23. The electrode 23 is connected to a power supply 25, while the outer the guide duct 21 is grounded. The particle separating duct 24 is formed with a plurality of particle admission holes 24a along an outer peripheral surface of the particle separating duct 24. The particle admission holes 24a are positioned at the same height and each of them is about 1 mm in diameter.
The operation of the apparatus for measuring the number of particles of the prior art so configured will be described. When particles are fed from the particle feeder 12, the particles are charged to a positive polarity by the neutralizer 14. The charged particles are introduced between the outer guide duct 21 and the inner guide duct 22. In the meantime, in order to smoothly deliver the charged particles to the inner guide duct 22, clean air is introduced into the inner guide duct 22. The particles charged into a polarity opposite to that of the electrode 23 move toward the electrode. Accordingly, charged particles of small size become attached to an upper portion of the electrode 23, and charged particles of large size move to a lower portion of the electrode 23. The charged particles moving downward are attached onto the lower portion of the electrode 23 or the charged particles of very large size, which are not attached onto the electrode 23, flow out of the guide duct 21. At this time, if some air is forcibly sucked through the particle separating duct 24, the particles of a predetermined size that reach the lower portion of the electrode 23 are introduced into the particle separating duct 24 through the particle admission holes 24a by the aforementioned operation of the air suction and then flow out. The charged particles flowing out by the particle separating duct 24 are within a predetermined size range. The selected charged particles are introduced into the particle counter 30 positioned downstream of the particle separator 20, and then, the number of the particles is measured.
Thus, by controlling a voltage applied to the electrode 23, the charged particles can be sorted according to size. If the above process is repeated with the voltage applied to the electrode 23 so controlled, it is possible to measure the number of particles according to size and then to obtain a size distribution of the whole particles.
However, the apparatus for measuring the number of particles of the prior art can only measure the number of the charged particles only within a specified size range by a single measurement. Thus, in order to obtain the numbers and the size distribution of whole particles, there is a disadvantage in that the measurement should be repeated a number of times with differing voltage applied to the electrode. Furthermore, when the particles in the air existing in a clean space such as a clean room are measured, the number of the particles in the clean space is fundamentally small. Thus, in practice, it is impossible to measure the numbers of charged particles according to size by changing voltage.